A wireless technology for isolating transmitting/receiving signals from an antenna in a co-channel has been studied for a long time in a repeater field. Repeaters can be classified into a mono-directional repeater, in which receiving and transmitting directions are different from each other, and a bi-directional repeater, in which receiving and transmitting directions are the same.
There should be a technological difference that antennas used in the co-channel mono-directional repeater are set up for different directivity, and antennas used in the bi-directional repeater are set up in such a manner that the entire or part of their directivity is overlapped.
The bi-directional repeater is a bi-directional wireless communication system. The bi-directional repeater receives a signal transmitted from a transmitting antenna in a repeater, restores amplitude of the signal, and transmits the signal through a co-channel in a region including the transmitting antenna. It is preferred to perform isolation based on a co-channel bi-directional wireless communication technology rather than a repeater technology since the transmitting antenna takes the received signal as receiving information and the signal can include speech or image information of a user.
An ultra isolation antenna suggested in the present invention is defined as an antenna capable of acquiring isolation more than a minimum level that can be used in a wireless communication field. Herein, the minimum isolation level is an isolation level for co-channel which is more than 120 dB in a mobile communication such as a cellular and a personal communication device.
When the co-time, co-channel and co-polarization bi-directional communication technology is realized based on a conventional isolation antenna technology, there is a problem that it is difficult to identify a transmitting signal and a receiving signal from each other since a reflected wave for a transmitting signal and a receiving signal are simultaneously transmitted from a receiving end to a receiver.
Conventional methods for solving the above problems are represented by two methods. One is a Frequency Division Duplex (FDD) method for performing communication by separating and using the transmitting frequency and the receiving frequency, that is, channels are set up differently. The other is a Time Division Duplex (TDD) method for separating and using transmitting time and receiving time. That is, the transmitting signal and the receiving signal are separated and used.
However, since the former method is not the co-channel bi-directional communication method and the latter method is not the co-time bi-directional communication method, there is a problem that communication capacity is reduced.
There is a technology for generating a transmitting signal and a receiving signal, whose polarizations are perpendicular to each other, by vertically setting two power feeders in a patch antenna, and maintaining isolation between the two feeders, as another conventional technology, which is not applied to an application system. The technology is proposed in an article by Karode, IEE National Conference on Antennas and Propagation, pp. 49-52, April 1999.
Also, Hao has suggested an isolation technology by changing polarization generation of a patch antenna applying a photo band gap (PBG) structure in an article, IEE, 11th International Conference on Antenna Propagation, pp. 86-89, April 2001.
However, as suggested in the result, isolation for a transmitting/receiving signal is very low in the same frequency. Thus, there is a problem that the above technology is not proper as an antenna for a co-channel bi-directional communication in diverse mobile communication, local communication, a broadcasting repeater and a satellite communication field requiring high isolation in the same frequency.
In the result of the conventional technologies suggested by Karodo and Hao, isolation of less than about 60 dB is acquired although transmitting/receiving frequency band or polarization is different.
Therefore, it is very difficult to realize a technology of an ultra isolation antenna which can be used in a co-channel bi-directional wireless communication system requiring ultra isolation more than 120 dB in the same polarization and same channel.